gsm_04_08: add function to get value string

The value string table gsm48_chan_mode_names[] lacks a function
to get the value string by a value. Lets add one.

Change-Id: I0757bcc278c140e18249e35864338e19cdaa3cf7
diff --git a/include/osmocom/gsm/protocol/gsm_04_08.h b/include/osmocom/gsm/protocol/gsm_04_08.h
index 4c98ff2..4635014 100644
--- a/include/osmocom/gsm/protocol/gsm_04_08.h
+++ b/include/osmocom/gsm/protocol/gsm_04_08.h
@@ -553,6 +553,8 @@
 };
 
 extern const struct value_string gsm48_chan_mode_names[];
+static inline const char *gsm48_chan_mode_name(enum gsm48_chan_mode val)
+{ return get_value_string(gsm48_chan_mode_names, val); }
 
 /* Chapter 9.1.2 */
 struct gsm48_ass_cmd {